Before making the decision to install a new roof over an existing one or to completely remove and replace the old one, there are a few critical considerations you need to address. The legality of roofing over your current roof is paramount. For instance, in Florida, this practice is no longer permitted due to code restrictions. While there may be certain areas where exceptions apply, generally speaking, code officials won't approve an overlay on the existing roof.
Assuming that adding a new layer atop your old roof is legal where you are, the next question is the condition of your current roof. Is it a suitable base for the new shingles? This is crucial because a roof overlay can impact the longevity of the new roofing system. The primary issue with a roof-over is that it hinders proper ventilation. For example, if your original roof is made of asphalt shingles or modified bitumen, the added layer can cause the material to heat up faster, accelerating the degradation process. As a result, your new roof might not last nearly as long as it should.
Of course, there are times when a roof overlay is a strategic choice. If you're focused on simply stating that your house has a "new" roof for selling purposes or other immediate needs, and the longevity of the new installation is not your primary concern, then overlaying could be a feasible route. It's often seen as an economical shortcut, as removing an entire roof can be a costly endeavor.
Ultimately, if the short-term benefits align with your current circumstances, and you're not overly concerned with the long-term lifespan of the roof, roofing over the old one could be the right decision for you at the time. However, it's important to carefully weigh the pros and cons, including the potential need for a sooner-than-expected replacement in the future, before making your final choice.